1. Field of the Invention
The present invention relates to an optical recording and pickup head for at least two optical discs in which recording and reproduction of information is performed by light of a respectively different wavelengths, and more particularly, to an optical recording and pickup head for recording information on a digital versatile disc (DVD) or a read-writeable compact disc (CD-RW) and reproducing information therefrom.
2. Description of the Invention
Optical discs are widely used as recording media for storing a large volume information. Among them, CDs and DVDs are widely being used. There are recordable CDs (CD-R) and read-writeable CD""s (CD-RW) as recent new CD types. As is well known, in case of DVDs, recording and reproduction of information is performed by laser light of 790 nm. Accordingly, an optical recording and pickup head which is compatible with both a DVD and a CD-RW includes two optical sources for emitting laser light of a respectively different wavelength, and an optical system for the two kinds of light.
Referring to FIGS. 1 and 2, a conventional optical recording and pickup head which is compatible with both a DVD and a CD-R will be described below.
FIG. 1 shows a configuration of a conventional optical recording and pickup head which is compatible with both a DVD and a CD-R. The optical recording and pickup head includes an optical source 1 emits first light of 660 nm in order to perform recording and reproduction of information with respect to a DVD 8, an optical source 11 emits second light of 790 nm in order to perform recording and reproduction of information with respect to a CD-R 9, and an objective lens 7 for focusing the first and second light emitted from the optical sources 1 and 11 on information recording surfaces of the DVD 8 and CD-R 9, respectively. A collimating lens 2 collimates the first light emitted from the optical source 1 into parallel light and transfers the collimated light to a beam splitter 3. The beam splitter 3 reflects the first light incident from the collimating lens 2 onto an interference filter prism 4. The interference filter prism 4 transfers the first light which is the parallel light incident from the beam splitter 3 to a quarter-wave plate 5. In more detail, the interference filter prism 4 is an optical device for total transmitting and total reflecting incident light according to its wavelength, in which the first light of 660 nm is totally transmitted and the second light of 790 nm incident from a converging lens 14 is totally reflected. A thin-film type variable aperture 6 transfers the first light incident from the quarter-wave plate 5 to the objective lens 7. The objective lens 7 focuses the parallel incident first light on the information recording surface of the DVD 8 whose thickness is 0.6 mm. As a result, the first light which is focused on and reflected from the information recording surface of the DVD 8 contains the information recorded on the focused position. The first light reflected from the information recording surface of DVD 8 transmits the objective lens 7, the variable aperture 6 and the quarter-wave plate 5, in turn and then incident to the interference filter prism 4. The interference filter prism 4 transfers the first light incident from the quarter-wave plate 5 to the beam splitter 3. The beam splitter 3 transfers the first light incident from the interference filter prism 4 to a photo detector 10. The photo detector 10 detects information from the first light incident from the beam splitter 3.
The second light of 790 nm emitted from the optical source 11 passes through a collimating lens 12 and a beam splitter 13 and then incident onto a converging lens 14. The converging lens 14 transfers the second light incident from the beam splitter 13 to the interference filter prism 4 in convergence light form. The interference filter prism 4 transfers the second light incident from the converging lens 14 to the quarter-wave plate 5 in divergence light form. The quarter-wave plate 5 transfers the second light incident from the interference filter prism 4 to the variable aperture 6. The variable aperture 6 transmits only a part of the second light incident from the quarter-wave plate 5 and transfers the transmitted second light to the objective lens 7 in divergence light form. The reason why the second light is incident to the objective lens 7 in diverging form is for focusing the second light on the information recording surface of the CD-R 9 without causing generation of spherical aberration.
FIG. 2 is a view for explaining the thin-film type variable aperture 6 of FIG. 1. The thin-film type variable aperture 6 has a structure of selectively transmitting the light incident to areas whose numerical aperture (NA) is less than or equal to 0.6. An region 1 is a region whose numerical aperture is less than or equal to 0.45, in which incident light of 790 nm and 660 nm is totally transmitted. An region 2 is a region whose numerical aperture ranges from 0.45 to 0.6, in which a dielectric thin film is coated and the light of 660 nm wavelength is totally transmitted and the light of 790 nm wavelength is totally reflected. The region 1 is made of a quartz (SiO2) thin-film in order to remove an optical aberration generated by the region 2 where the dielectric thin-film is coated. The variable aperture 6 having the transmission characteristic totally transmits the first light of 660 nm wavelength irrespective of the region and totally transmits the second light of 790 nm wavelength which is incident into the region 1 whose numerical aperture is less than 0.45 to be transferred to the objective lens 7, and totally reflects the second light which is incident into the region 2 whose numerical aperture is more than or equal to 0.45. Thus, the numerical aperture with respect to the light incident to the objective lens 7 is limited according to its wavelength.
The objective lens 7 focuses the second light incident from the thin-film type variable aperture 6 on the information recording surface of the CD-R 9 whose thickness is 1.2 mm, thereby forming an optical spot. The second light reflected from the information recording surface of the CD-R 9 passes the objective lens 7, the variable aperture 6 and the quarter-wave plate 5, in turn and then incident to the interference filter prism 4. The interference filter prism 4 reflects the second light incident from the quarter-wave plate 5 to a converging lens 14. The converging lens 14 transfers the second light to a beam splitter 13. The beam splitter 13 transfers the second light incident from the converging lens 14 to a photo detecter 15. The photo detector 15 receives the second light from the beam splitter 13 and detects information from the received second light. Thus, the optical recording and pickup head of FIG. 1 can perform recording and reproduction of information with respect to both the DVD 8 and the CD-R 9.
However, the optical recording and pickup head of FIG. 1 should include a particular variable aperture 6 in order to selectively limit the numerical aperture with respect to the light incident to the objective lens 7 according to the wavelength of the incident light. Since a quartz thin film is coated on the region 1 of the variable aperture 6 and a multilayer dielectric thin film having the thickness of micrometer unit is configured on the region 2 thereof, the manufacturing process is complicated and the production cost becomes high. Also, since the second light for use in a CD-R which is incident to the region whose numerical aperture is more than or equal to 0.45 is totally reflected, it is not appropriate for adapting itself to an optical system of an optical recording and pickup head for use in a CD-RW requiring a larger numerical aperture of about 0.5 or more and a higher optical efficient for recording.
To solve the above problems, it is an object of the present invention to provide an optical recording and pickup head for use in both a DVD and a CD-RW, including a flat plate lens for totally transmitting incident light according to its wavelength and diffracting the same toward an optical axis of an objective lens.
To accomplish the object of the present invention, there is provided an optical recording and pickup head for a plurality of optical discs for performing recording and reproduction of information with respectively different wavelengths, the optical recording and pickup head comprising: a first optical source for emitting first light having a relatively short wavelength; a second optical source for emitting second light having a relatively long wavelength; a photo detector; an objective lens for focusing the light emitted from the first and second optical sources on the information recording surfaces of the optical discs, respectively; an optical path altering unit for transferring the light emitted from the first and second optical sources to the objective lens and transferring the first and second light reflected form the information recording surfaces of the plurality of discs to the photo detector, respectively; and a flat plate lens for substantially totally transmitting the first light proceeding from the optical path altering unit to the objective lens and diffracting the second light proceeding from the optical path altering unit to the optical axis of the objective lens.